You have toured the endomembrane system and learned where mitochondria and chloroplasts came from. Step back and ask why a eukaryotic cell is divided into so many membrane-bound rooms. The answer is that different jobs need different conditions, and walls let them happen side by side.
Key idea
Membranes divide a eukaryotic cell into compartments with their own conditions (pH, ions, enzymes), so reactions that would interfere can run at the same time. Membranes also provide surface where enzymes line up in order, as in the folded inner membrane of a mitochondrion and the thylakoids of a chloroplast.
| Compartment | Its special conditions | What that allows |
|---|---|---|
| Lysosome | Acidic inside (about pH 5); digestive enzymes | Breaks down worn-out parts without digesting the cytoplasm (near pH 7) |
| Nucleus | DNA and RNA processing enzymes | mRNA is processed before ribosomes reach it |
| Mitochondrion | Inner membrane folded into cristae; an H⁺ gradient | Chemiosmosis makes ATP |
| Chloroplast | Thylakoid membranes in stacks; stroma around them | Light reactions and the Calvin cycle in separate spaces |
| Rough ER and Golgi | Enzymes that fold and tag proteins | Proteins for export are made and shipped apart from the cytoplasm |
Why it works
A membrane is a barrier that most ions and large molecules cannot cross without a transport protein. So a cell can pump H⁺ into a lysosome and keep it there, or concentrate one enzyme in one place. The enzymes of a lysosome work best in acid, so if a lysosome bursts, its enzymes do little harm in the neutral cytoplasm. The wall protects twice: it keeps the enzymes in, and the conditions outside keep any escapees weak.
Worked examples
Worked example: Why pH matters
A lysosome enzyme has its highest activity at pH 5 and almost none at pH 7. What does that mean for a cell if a few lysosomes leak?
- Inside the lysosome (pH about 5) the enzyme works well.
- The cytoplasm is near pH 7, where the enzyme hardly works.
- Leaked enzymes are mostly inactive.
Answer: The cell is protected: the enzymes that escape do little damage in the neutral cytoplasm.
Worked example: A cell with no organelles
A bacterium has no mitochondria. Where does it run its electron transport chain?
- An electron transport chain must sit in a membrane so it can build an H⁺ gradient across it.
- The bacterium has one membrane that encloses its cytoplasm: the plasma membrane.
- Its chain pumps H⁺ out across that membrane, and ATP synthase there lets H⁺ back in.
Answer: In its plasma membrane. Some bacteria, like cyanobacteria, also have folded inner membranes (thylakoids) for photosynthesis.
Common mistake
Saying prokaryotes have no compartments at all. They lack membrane-bound organelles, but they use their plasma membrane, and in some species folded internal membranes, for the same jobs. The difference is in how many separate spaces a cell has, not whether membranes do work.